During conventional drilling, after an inner tube of a head assembly is full of a sample, an overshot assembly is lowered (or pumped) toward the bottom of a drill hole to retrieve the head assembly. Conventional overshot assemblies include heavy-duty lifting dogs that are configured to securely grab a spearhead (spearpoint) that is coupled to the proximal end of the head assembly. After engagement between the lifting dogs and the spearhead, the overshot is retrieved from the drill hole, and the sample is extracted from the inner tube.
Spearheads and locking dogs are typically formed by a casting process. Due to the nature of the casting process, the material of the spearhead and locking dogs is typically of reduced quality, more easily distorted, and less wear-resistant when compared to machined materials. Additionally, existing spearheads and locking dogs only function together within a narrow range of relative orientations. Due to these limitations, it can be challenging to achieve proper engagement between existing spearheads and locking dogs when conditions within the drill hole are not ideal.
Some recent overshot assemblies have been designed to address one or more of the above-identified issues. However, these overshot assemblies are mechanically complex, with a large number of parts, and can be difficult to install and/or assemble. Additionally, these overshot assemblies are likely to experience undesired corrosion.
Accordingly, there is a need in the pertinent art for an overshot assembly that is easier to install and assemble and more robust, reliable, and corrosion-resistant than existing overshot assemblies. There is a further need in the pertinent art for an overshot assembly that retains these properties over a wide range of angular orientations.